Overhead cranes which travel on wheels along spaced apart, generally parallel rails are subject to the continuous problem of the skewing of the crane on the rails. The forces causing skewing are due to rail displacement caused by rail support changes, rail deterioration resulting from improper adjustment of acceleration and deceleration forces of drive motors and brakes, and variations in traction due to rail contamination from moisture vapor and airborne particles. The skewing itself exacerbates the problem since it produces stresses on the rail structure which contribute further to the displacement of the rails. Moreover, the skewing causes severe stressing and wear of the crane wheels. The end result of rail displacement and deterioration and consequent increased skewing is a short wear life of the rails requiring their relatively frequent replacement and very frequent replacement of the wheels.
Various prior art solutions to the skewing problem have been developed. These include controls in which a sensing device is used for detecting skew and adjusting the drive motors of the crane to correct the skew. Another approach, upon sensing skew of the bridge, is to either apply a friction drag to the leading skewed end of the bridge or activate a wheel brake on the leading drive wheel of the skewed bridge. A further solution, disclosed in U.S. Pat. No. 3,095,829, in a crane having drive wheels driven and controlled independently, is to decrease the clearance between the rail and the outside flange of each of the drive wheels. Consequently, the outside flange of the leading drive wheel, when the crane moves to a skewed position, will contact the outer side of the rail on which it rides and cause that wheel as well as its drive system to slow down due to the resulting friction and thereby correct the skew. The skew sensing devices used in prior art skew correction methods have typically ben contacting devices such as rollers which are connected to switches and proximity type switches which will provide an output signal indicative of their distance from the rail.
U.S. Pat. No. 5,080,021 discloses an apparatus for correcting skew of a traveling crane operating on spaced apart rails in which the drive wheels always rotate at the same speed.